1. Field of the Invention
The present invention generally relates to a Super-conducting Quantum Interference Device (SQUID), and more particularly, to a reference-current optimizing apparatus of a double relaxation oscillation SQUID.
2. Background of the Related Art
In general, a SQUID refers to a high-sensitivity magnetometer that can detect up to a minute magnetic flux density of a 10−15 T (tesla) scale by employing the Josephson effect (i.e., an effect in which when electrons pairs are tunneled between two super-conducting rings, the super-conductivity of the super-conducting ring junction collapses at a critical current value and the magnetic flux enters the super-conducting ring accordingly). More particularly, a Double Relaxation Oscillation SQUID (hereafter, referred to as “DROS”) has a great magnetic flux-voltage characteristic and a high conversion coefficient, and it can detect a rapid magnetic flux signal using a simple pre-amplifier.
FIG. 1 shows a current source for driving a Reference Junction DROS (hereinafter, referred to as “RJ-DROS”) and an overall construction for detecting the output of the DROS.
A RJ-DROS 101 includes a signal SQUID 102 and a reference junction 103. The RJ-DROS 101 is disadvantageous in that it has a fixed reference current since the reference junction is used, but is advantageous in that it has a simple structure and convenient operation in comparison with a Reference SQUID DROS (hereinafter, referred to as “RS-DROS”) having two SQUIDs.
An external magnetic flux reacts through an input coil Mi 104 of the signal SQUID 102 and a feedback coil Mf 105 is used when the DROS operates in Flux-Locked Loop (FLL) mode. An application current Ib for driving the RJ-DROS 101 is controlled using an application current control current source 106 at normal temperature. A reference current of the reference junction 103 is controlled using a reference current control current source 107. The output of the RJ-DROS 101 is detected by a preamplifier 108 at both ends of the reference junction 103.
FIG. 2 is a circuit diagram of a fixed current application circuit used to detect the output of the DROS that reacts to variation in the magnetic flux more accurately.
As shown in FIG. 2, in the fixed current application circuit, input terminals 205, 206 of a preamplifier having a plurality of junction bipolar transistors 201 are connected to the reference junction 103. To remove the input application current of the preamplifier, the fixed current application circuit includes a fixed voltage source 202 and fixed application resistors Rb 203, 204.
In the related art DROS constructed above, a current must flow from the outside in order to control the reference current flowing through the reference junction. To this end, another electric wire through which the current can flow is further required. If another electric wire is added as described above, the number of connection lines are increased in the multi-channel system. Accordingly, problems arise because not only a thermal conduction phenomenon is generated, but also the fabrication of a low temperature insert system is complicated.
Meanwhile, the preamplifier having the plurality of junction bipolar transistors 201 with a low voltage noise is used in order to measure the output of the RJ-DROS. In this case, if the input application current of the preamplifier flows into the reference junction 103, the DROS may not operate or the characteristics of the DROS are changed, exceeding the optimal operating conditions. This hinders a stabilized DROS operation.
To avoid the problem, the fixed current application circuit including the fixed voltage source 202 and the fixed application resistors Rb 203, 204 are used, as shown in FIG. 2. However, an application current generated from the fixed application current circuit is not constant because of the difference in the properties of electrons components of the circuit and heat generated from peripheral circuits. This causes the current to flow into the input terminals 202 of the reference junction 103. As a result, there is a possibility that the DROS may not operate or does not operate under optimal conditions as described above.